In the prior art electromotive grinding machine, a motor is mounted therewithin. The driving shaft of the motor is combined with an output shaft. The output shaft will extend outwards. The output shaft is further combined with a grinding element. The front surface of the grinding element is combined with a cutting tool, for example a emery cloth. When the grinding element is driven to rotate by the motor, the grinding element will grind the surface of a working piece to attain a required finishing level. In general, the rotary speed is very rapid. However, a higher actuating speed is often dangerous to users and a very high speed grinding is not practical. Therefore, other than installing a retarding gear outside the casing, there are no retarding methods in the prior art. However, the casing only has a finite space, and the retarding gear not only occupies much space, but also causes a complex structure and increases the manufacturing cost. Moreover, the retarding gear will increase the whole weight.